In recent years, it has been demanded that recording materials as well as inking materials, which are employed in printers, printing presses, markers, and writing means be free of organic solvents and be comprised of water. Specifically, widely employed as water-based recording materials are ones which are mainly comprised of an aqueous water-soluble dye solution and of a dispersion of fine pigment particles.
Employed as water-based ink utilizing water-soluble dyes is one which is prepared by adding moisture retention agents such as glycols and alkanolamines, and surface active agents to control surface tension, and if desired, viscosity increasing agents (or thickeners) to an aqueous solution of water-soluble dyes which are classified into acidic dyes, direct dyes, and some food dyes. Water-based ink employing said water-soluble dyes is most commonly employed due to its high reliability for minimized clogging at writing tips or recording systems. However, water-based ink employing such water-soluble dyes tend to result in bleeding on recording paper sheets due to the fact that the employed dyes are water-soluble. As a result, their use is limited and the recording quality inevitably degrades. In addition, it is difficult to assume that water-soluble dyes, which simply penetrate into a paper sheet, retained and dried, are subjected to “dying”. As a result, the resulting light fastness is not fully acceptable.
As methods to overcome said drawbacks of water-based ink prepared by utilizing water-soluble dyes, addition of fine resinous particles such as emulsion and latex have been investigated over an extensive period. Japanese Patent Publication Open to Public Inspection No. 55-18418 proposes an ink jet recording material to which a latex is added which is “one type of a colloid composition prepared by dispersing a component such as rubber and resins into water in the form of fine particles (having a particle diameter of about 0.01 to several μm) employing an emulsifier.” In order to improve light fastness as well as to minimize effects such as bleeding by the addition of said latex as proposed by said patent, the necessary amount of said latex is larger than the amount of used dyes. As a result, it becomes very difficult to assure dispersion stability as well as ejection stability.
In order to solve problems with low water fastness as well as low light fastness of the water-based ink comprising said water-soluble dyes, several proposals have been made for ink jet recording ink in which water-dispersible resins are colored with oil-soluble dyes or hydrophobic dyes. For instance, Japanese Patent Publication Open to Public Inspection Nos. 55-139471, 58-45272, 3-250069, 8-253720, 8-92513, 8-183920, and 2001-11347 propose ink comprising emulsion polymerization particles dyed with oil-soluble dyes or dispersed polymerization particles. In water-based ink employing such fine colored particles, when said dyes are present on the surface of said particles or in the exterior of said particles, the desired effects are not fully realized. As a result, it is hard to improve various kinds of performance such as dispersion stability, ejection stability, and light fastness.
On the other hand, with regard to pigments, in order to improve dispersion stability, ejection stability, and lightfastness, tried has been covering pigment particles with film forming resins other than dispersing agents. For example, Japanese Patent Publication Open to Public Inspection Nos. 8-269374, 9-151342, 10-88045, and 10-292143 describe examples in which pigment particles are covered with resins. However, it is not easy to totally cover minute pigment particles with resins. Currently, covered pigment particles have not yet been developed which exhibit superior performance compared to conventional pigment dispersions.
As noted above, a water-based ink, comprising fine colored particles employing oil-soluble dyes or pigments, has hidden potential to realize high recording quality while overcoming said drawbacks. However, various problems are left unsolved and improvements are still sought.